The present invention relates to a zoom lens system adapted for use in a compact camera and which is subject to no strict condition on back focus.
The following three types of zoom lens systems are available for use in compact cameras.
(I) A telephoto two-component type composed of a first lens component having a positive focal length inclusive of a stop diaphragm and a second lens component having a negative focal length. An example of this type of lens system is disclosed in Japanese Unexamined Published Patent Application Nos. 128911/1981 and 48009/1985.
(II) A three component type which is a modification of type (I) and which is composed of a first lens component having a positive focal length inclusive of a diaphragm stop, a second lens component having a positive focal length, and a third lens component having a negative focal length. An example of this lens type is disclosed Japanese Unexamined Published Patent Application No. 184916/1983.
(III) A four-component type composed of a first lens component having a positive focal length, a second lens component having a negative focal length, a third lens component having a positive focal length, and a fourth lens component having a negative focal length. An example of this lens type is disclosed in Japanese Unexamined Published Patent Application No. 57814/1985.
These zoom lens systems, however, have the following problems:
The two-component type (I) lens system has a very simple construction but it requires a complicated camera composition since the first lens component including a diaphragm stop has to be displaced by ten-odd millimeters and the large-diameter second lens component by about 30 mm in order to achieve zooming. These large amounts of displacement also make it difficult to realize a compact camera.
The three-component type (II) lens system only differs from type (I) in that the first lens component is divided into two positive lens units, so the first lens component including a diaphragm stop has to be displaced by the same about as what is necessary in type (I) for zooming. In addition, the large-diameter third lens component has to be moved by an amount that is not much different from what is required for the second lens component of type (I). It is therefore also difficult to realize a compact camera.
The four-component type (III) lens system employs an entirely different lens composition than either of types (I) and (II) in that the second lens component including diaphragm stop is fixed. As for the other lens components, the first lens component needs to be displaced by the same amount as what is necessary in type (I) but the third and fourth lens components are displaced by smaller amounts, the amount of displacement of the large-diameter fourth lens component being only about half the amount required in type (I). However, the use of four lens components renders the lens configuration very complex. In addition, the individual lens components, in particular, the second and third components have such large powers that any slight error introduced in the manufacturing process will cause serious effects on the performance of the resulting lens system. Therefore, this type of lens system is rather difficult to manufacture.